Positive electrode active material for secondary battery and fluoride ion secondary battery using said positive electrode active material

Abstract

To provide a cathode active material having high safety enough to avoid firing an electrolyte solution, which allows a satisfactory high output voltage to be achieved, which can be charged even in a state immediately after being manufactured, and which can be composed of a commercially available anode material, and a fluoride ion secondary battery using the active material.SOLUTION: A cathode active material for a secondary battery and a fluoride ion secondary battery using the same are disclosed. The cathode active material for a secondary battery comprises a metal complex represented by the general formula, MxMo(CN)8 yH2O, where M is one kind selected from a group consisting of Fe, Co, Ni and Zn; x and y satisfy 1.95<x<2.05 and 0≤y≤9, respectively. The cathode active material is in a state of being able to be oxidized, namely a state of being able to be charged in an electrochemical reaction where fluoride ions functions as insertion species.SELECTED DRAWING: Figure 1(A)

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a secondary battery and a fluoride ion secondary battery using the positive electrode active material. [Background technology]

[0002] Lithium-ion secondary batteries have a high voltage per cell and a high energy density, and are therefore used in a wide range of applications, including portable electronic devices, electric vehicles, and power storage batteries. Lithium-ion secondary batteries are secondary batteries that utilize the reaction between lithium ions and a positive electrode active material and the reaction between lithium ions and a negative electrode active material. Similarly, development of sodium-ion secondary batteries and potassium-ion secondary batteries is underway as cation-based batteries that use alkali metal ions as carriers, using sodium and potassium, which are alkali metals like lithium. In lithium-ion secondary batteries, a principle called intercalation reaction is used in the electrodes to accommodate the lithium ions that move back and forth between the two electrodes, the positive and negative electrodes. In intercalation reaction batteries, lithium ions are inserted into the gaps in the crystalline structure of the active material and stored. The electrodes themselves do not change much even after repeated charging and discharging, so phenomena such as electrode deformation caused by repeated charging and discharging, which occurred in conventional electrode-dissolving secondary batteries, are suppressed. Taking electrode deformation into account, the distance between the electrodes, which was set larger than necessary, can be reduced, allowing the battery itself to be designed more compactly. Prior art

[0003] [ Lithium-ion secondary batteries and cyanometallates ] Metal complexes in which metal ions are bridged with cyano groups (hereafter referred to as cyanometallates) are promising candidates for intercalation in the positive electrodes of lithium-ion, sodium-ion, and potassium-ion batteries. This is because the bridged metal ions form a three-dimensional structure resembling a jungle gym (Fig. 1(A)), allowing charge carriers to be inserted into the gaps in the jungle gym structure.

[0004] For example, Patent Document 1 discloses a compound of general formula A consisting of an iron-based cyanometallate. x Patent Document 1 discloses a battery cathode material represented by Mn[Fe(CN)6]y·zH2O, where A is an alkali cation or alkaline earth cation, x = 1 to 2, y = 0.5 to 1, and z = 0 to 3.5. x The charge and discharge characteristics of a secondary battery fabricated using a positive electrode containing Mn[Fe(CN)6]y·zH2O, an organic electrolyte containing A, and a negative electrode are described. According to the description, the charge and discharge characteristics show a single plateau discharge curve where the output voltage is nearly constant, and this single plateau discharge curve covers a range of 85% to 15% of the total discharge capacity. Regarding the discharge characteristics, at the 85% point of the single plateau curve, the secondary battery had a discharge capacity exceeding 90 milliampere-hours per gram of positive electrode active material (mAh / g). A is sodium, and Na x Mn[Fe(CN)6] y Using zH2O as the positive electrode active material, a secondary battery containing an organic electrolyte and a negative electrode was fabricated. A large discharge capacity of 170 milliampere-hours per gram (mAh / g) was obtained, with an output voltage of 2-4.2V.

[0005] [ Increasing the output voltage of lithium-ion secondary batteries: The first problem with lithium-ion secondary batteries ] Increasing the output voltage is an effective way to increase the energy density of a battery. In theory, a higher output voltage can be obtained by increasing the difference in redox potential between the materials used in the positive and negative electrodes. This can be achieved by using a material with a low redox potential for the negative electrode and a material with a high redox potential for the positive electrode.

[0006] In the invention disclosed in Patent Document 1, the following oxidation-reduction reaction of iron complex ions or manganese complex ions coordinated with a cyano group is used as a positive electrode active material, and is responsible for the charge-discharge reaction. [M III (CN)6] 3- + e - ⇔ [MII (CN)6] 4- (wherein M is Fe or Mn) In this reaction, the higher the redox potential of M(CN)6, the better. However, as disclosed in Non-Patent Document 1, when M is iron (Fe), the redox potential is +0.355 V (vs. standard hydrogen electrode potential), and when M is manganese (Mn), the redox potential is −0.240 V (vs. standard hydrogen electrode potential), which is not particularly high compared to that of other cyanometallates.

[0007] In order to improve the redox potential, it is considered to use molybdenum (Mo) as M. The reason is that the redox reaction of the molybdenum complex ion coordinated with a cyano group represented by the following formula is [Mo V (CN)8] 3- + e- ⇔ [Mo IV (CN)8] 4- This is because, as disclosed in Non-Patent Document 2, the oxidation-reduction potential of the iron complex ion (Mo) is +0.725 V (vs. standard hydrogen electrode potential), which is a higher potential than the oxidation-reduction potentials of the iron complex ion and manganese complex ion. Therefore, if molybdenum (Mo) is used for M, the output voltage of the secondary battery can be increased compared to when iron (Fe) or manganese (Mn) is used, and as a result, a secondary battery with a high energy density can be obtained. Therefore, A x If it were possible to synthesize a material such as Mn[Mo(CN)8]y·zH2O (where A is an alkali cation or alkaline earth cation such as lithium Li, sodium Na, or potassium K), it would be possible to expect an improvement in output voltage by applying this directly to a secondary battery with the configuration shown in Patent Document 1. The applicant conducted an intensive search for such a material, but was unable to find a substance that contains the structure [Mo(CN)8], A, and Mn.

[0008] However, Non-Patent Document 3 lists [Mn(HO)] [Mn(HCOO) as an example of a cyanate salt containing pentavalent molybdenum. 2 / 3 (H2O) 2 / 3 ] 3 / 4[Mo(CN)8]·H2O is disclosed, and it is shown that lithium ions and sodium ions can be reversibly inserted and extracted into this material. Non-Patent Document 3 does not show any prototype secondary battery examples, but the material contains a molybdenum complex ion framework [Mo(CN)8] structure, which is key to improving output voltage, and is capable of intercalation reactions of lithium ions and sodium ions. Therefore, if this molybdenum-based cyano acid salt is applied to a secondary battery with the configuration shown in Patent Document 1, it may be possible to realize a lithium-ion secondary battery with a high output voltage.

[0009] However, [Mn(HO)] [Mn(HCOO) 2 / 3 (H2O) 2 / 3 ] 3 / 4 [Mo(CN)8]·H2O has some drawbacks. First, the raw materials for molybdenum cyanates are generally obtained as compounds containing tetravalent molybdenum, so cyanates containing pentavalent molybdenum require a complicated synthesis procedure, such as using raw materials that have been pre-oxidized. Second, since the above materials do not contain lithium ions, sodium ions, or potassium ions at the time of their production, when the above molybdenum cyanates are used as a positive electrode active material, the initial reaction inevitably involves the insertion of lithium ions, sodium ions, and potassium ions into the material, and at the same time, molybdenum accepts electrons from the electrode side to convert from pentavalent to tetravalent ([Mo V (CN)8] 3- → [Mo IV (CN)8] 4- ) which means that the first reaction is discharge. In this case, the negative electrode must also be in a dischargeable state (reduced state). Examples of dischargeable negative electrode materials include graphite that occludes lithium ions, sodium ions, and potassium ions. However, such compounds are not normally available on the market and are unstable in the atmosphere. Negative electrode active materials normally available on the market are limited to substances in an oxidized state (rechargeable state). Therefore, the [Mn(HO)][Mn(HCOO) 2 / 3 (H2O) 2 / 3 ] 3 / 4It is difficult to fabricate a lithium-ion secondary battery using [Mo(CN)8]·H2O.

[0010] As described above, it is believed that it is theoretically possible to fabricate a lithium ion secondary battery using a cyano acid salt containing a molybdenum complex ion skeleton as a positive electrode active material. However, there are many challenges to actually producing such a battery, such as the need for anode materials that are difficult to obtain and unstable in the atmosphere.

[0011] Although it is known that the use of a molybdenum complex ion framework [Mo(CN)8] could potentially increase the voltage of lithium-ion secondary batteries, the fact that this has not yet been realized is the "first problem" with lithium-ion secondary batteries.

[0012] [ Safety of Lithium-ion Secondary Batteries: The Second Problem with Lithium-ion Secondary Batteries ] Furthermore, lithium-ion secondary batteries have the unavoidable problem of low safety due to their structure and physical properties. This problem stems from the fact that they must use organic electrolytes, which can be flammable.

[0013] Essentially, only organic electrolytes can be used in lithium-ion secondary batteries because, when aqueous electrolytes are used, the reaction of reducing water to generate hydrogen is more likely to occur than the reaction of reducing lithium ions to be absorbed into the negative electrode, preventing lithium from being absorbed into the negative electrode and preventing the exchange of charge between the electrode and the electrolyte using lithium ions.

[0014] Next, we will explain the causes of organic electrolyte ignition. When a lithium-ion secondary battery is overcharged, metallic lithium that can no longer be absorbed in the negative electrode can precipitate on the negative electrode. If metallic lithium grows across the separator and reaches the positive electrode, an internal short circuit occurs within the battery, which generates heat. This heat can cause the organic electrolyte to ignite.

[0015] Furthermore, when oxide-based materials are used as the positive electrode active material, oxygen atoms in the positive electrode active material are known to be released at high temperatures. The released oxygen atoms are highly reactive and may react with the organic electrolyte, causing combustion.

[0016] As described above, although lithium ion secondary batteries have extremely high performance, they have unavoidable problems with respect to safety. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] International Publication No. 2014 / 118854 [Patent Document 2] Japanese Patent Application Publication No. 2019-204775 [Patent Document 3] Japanese Patent Application Publication No. 2017-220301 [Non-patent literature]

[0018] [Non-Patent Document 1] Handbook of Chemical Equilibria in Analytical Chemistry, Wiley, New York,1985. [Non-patent document 2] Electrochemistry Handbook, 5th Edition, Maruzen, Tokyo, 2020 [Non-patent document 3] Inorganic Chemistry, vol.52, pp.3772-3779, 2013. Summary of the Invention [Problem to be solved by the invention]

[0019] [ Anion-based secondary batteries: How to simultaneously solve the first and second problems ] The present applicant has conducted extensive research into the above two problems that have been associated with conventional lithium ion secondary batteries, and as a result has discovered a method for simultaneously solving the first and second problems.

[0020] That is, the carrier is an anion instead of a cation, and an aqueous electrolyte is used.

[0021] The first problem is that the initial reaction is a discharge. In order for the reaction in which charge carriers are inserted into the positive electrode to be a charge in a secondary battery, the charge carriers should be anions rather than cations.

[0022] The second problem is the low safety of lithium-ion secondary batteries. This is due to the fact that the charge carriers are cations. In other words, when the charge carriers are cations, the charge carriers are lithium ions, sodium ions, potassium ions, etc., so organic solvents must be used in the electrolyte, which poses a risk of burning organic solvents. If the carrier is selected from anions rather than cations and an aqueous electrolyte can be used, it may be possible to avoid the electrolyte from igniting.

[0023] [ Fluoride-ion secondary battery ] Fluoride ion secondary batteries, which utilize the reaction of fluoride ions, have been studied as anion-based secondary batteries that use anions as charge carriers. Generally, anions have a large ionic radius and are considered unsuitable for intercalation reactions. However, fluoride ions, the smallest stable anions, have an atomic radius comparable to that of sodium and potassium ions, making them the only anions that can freely enter and exit the gaps in the molecular structure of intercalation electrodes. The use of fluoride ions may enable the realization of anion-based intercalation electrodes.

[0024] Patent Document 2 discloses metal fluorides and metal oxyfluorides containing at least one metal selected from the group consisting of alkali metals, alkaline earth metals, scandium, yttrium, and lanthanoids, a first transition metal, a second transition metal different from the first transition metal, and fluorine as positive electrode materials for fluoride ion batteries. Patent Document 2 describes that when such a positive electrode active material is used, a large discharge capacity of 165 milliampere-hours per gram (mAh / g) is obtained, and speculates that an intercalation reaction may occur in the electrode as the carrier storage mechanism.

[0025] In addition, Patent Document 3 discloses a metal oxyfluoride A having a layered perovskite structure. n+1 B n O 3n+1-α F x The present application discloses a battery cathode material comprising: where A is at least one of an alkaline earth metal element and a rare earth element, B is at least one of Mn, Co, Ti, Cr, Fe, Cu, Zn, V, Ni, Zr, Nb, Mo, Ru, Pd, W, Re, Bi, and Sb, n is 1 or 2, 0≦α≦3.5, and 0≦x≦5.5. Patent Document 3 discloses that when the active material is charged, fluoride ions are inserted by an intercalation reaction, and when charging is continued, some of the O elements are replaced by F elements, resulting in a new high-potential electrode reaction. Furthermore, it is disclosed that as a result of this two-stage electrode reaction, the charge capacity becomes 225 milliampere-hours per gram (mAh / g).

[0026] The secondary batteries disclosed in Patent Documents 2 and 3 disclose that fluoride ions are inserted into the positive electrode by an intercalation reaction, but the positive electrode does not contain a cyanometallate, and naturally, the output voltage is not improved by using a molybdenum complex ion framework [Mo(CN)8].

[0027] Furthermore, the secondary batteries disclosed in Patent Documents 2 and 3 use metal fluorides as the positive electrode active material, which has the disadvantage in use that they absorb moisture in the air and easily decompose. Furthermore, the synthesis of the material requires many steps, such as combustion in a fluorine gas stream or in an inert atmosphere.

[0028] In particular, the metal oxyfluoride disclosed in Patent Document 3 contains oxygen, and furthermore, an organic solvent is used in the electrolyte, which may cause combustion or explosion. In other words, no solution to the second problem mentioned above is recognized.

[0029] As described above, although intercalation-type secondary batteries using fluoride ions as charge carriers have been disclosed in the prior art secondary batteries, none of them simultaneously solves the first and second problems described above.

[0030] As a result of extensive research, the present inventors have succeeded in solving the first and second problems by using a positive electrode containing a molybdenum-based cyanometallate having a molybdenum complex ion framework [Mo(CN)8] and using fluoride ions as a carrier. That is, they have succeeded in providing a positive electrode active material that can obtain a satisfactorily high output voltage, can be charged in a state immediately after manufacture, can be constructed using commercially available negative electrode materials, and has high safety by preventing the electrolyte from igniting, as well as a fluoride ion secondary battery that uses the positive electrode active material. [Means for solving the problem]

[0031] The invention according to claim 1 is a compound having the general formula M x Mo(CN) 8·A metal complex represented by yH2O, wherein in the general formula, M is one selected from the group consisting of Fe, Co, Ni, and Zn, x and y satisfy the relationships 1.95 < x < 2.05 and 0 ≦ y ≦ 9, and it is in an oxidizable state, that is, a chargeable state, in an electrochemical reaction using fluoride ions as an insertion species. It relates to a positive electrode active material for a secondary battery.

[0032] The invention according to claim 2 is a fluoride ion secondary battery using the positive electrode active material for a secondary battery according to claim 1, wherein the fluoride ion secondary battery includes a positive electrode, a negative electrode, an electrolytic solution, and a separator. The positive electrode contains the positive electrode active material for a secondary battery, the electrolytic solution is an aqueous solution in which a fluoride salt is dissolved, and the separator is provided between the positive electrode and the negative electrode. It relates to a fluoride ion secondary battery characterized by this.

[0033] The invention according to claim 3 relates to the fluoride ion secondary battery according to claim 2, characterized in that the battery is a sealed type battery prepared by infiltrating the electrolytic solution into the separator.

[0034] The invention according to claim 4 relates to the fluoride ion secondary battery according to claim 2 or 3, characterized in that the fluoride salt is one or more selected from lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride.

Effects of the Invention

[0035] According to the invention according to claim 1, the general formula is M x Mo(CN) 8·A metal complex represented by yH2O, wherein in the general formula, M is one selected from the group consisting of Fe, Co, Ni, and Zn, x and y satisfy the relationships 1.95 < x < 2.05 and 0 ≦ y ≦ 9, respectively, and it is in an oxidizable state, i.e., a chargeable state, in an electrochemical reaction using fluoride ions as an insertion species. By fabricating a fluoride ion secondary battery using a positive electrode containing a positive electrode active material for a secondary battery, the first problem and the second problem that the conventional lithium ion secondary battery had can be solved. That is, a satisfactorily high output voltage can be obtained, the positive electrode of the fluoride ion secondary battery is chargeable immediately after manufacture, thus a negative electrode can be constructed using a negative electrode material that is commercially available, and a positive electrode active material necessary for fabricating a secondary battery having high safety that can avoid explosive combustion and ignition of the electrolytic solution can be provided. In the present invention, when the positive electrode active material is in a "chargeable state", it means that there is an excess portion in the voids of the crystal structure in the positive electrode active material into which anions such as fluoride ions can be inserted for the charge compensation reaction accompanying the oxidation reaction of the positive electrode active material.

[0036] The fluoride ion secondary battery having the configuration according to claim 2 includes a positive electrode, a negative electrode, an electrolytic solution, and a separator. Since the positive electrode contains the positive electrode active material for the secondary battery, it can output a satisfactorily high voltage. Also, since the electrolytic solution dissolves a fluoride salt, the first reaction is charging. Thus, it can be configured using a negative electrode material that is commercially available. Furthermore, since the fluoride ion secondary battery according to claim 2 uses water as a solvent, it is not necessary to use a non-aqueous electrolytic solution such as an organic electrolytic solution. Therefore, even if heat generation occurs by chance, the electrolytic solution will not catch fire and the safety is high.

[0037] According to the invention according to claim 3, there is provided a highly practical and compact fluoride ion secondary battery, which is a sealed battery prepared by infiltrating the electrolytic solution into the separator.

[0038] According to the invention of claim 4, the fluoride salt of the electrolyte is one or more selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride. These fluoride salts are readily available on the market and are stable in the atmosphere. [Brief explanation of the drawings]

[0039] [Figure 1(A)] FIG. 1(A) is a schematic diagram showing the molecular structure of a metal complex according to the present invention, which is represented by the general formula MxMo(CN)8·yH2O. [Figure 1(B)] FIG. 1(B) is a schematic diagram showing a charged state in which fluoride ions are inserted into the positive electrode containing the positive electrode active material of the fluoride ion secondary battery according to the present invention. [Figure 2(A)] FIG. 2(A) is a schematic diagram showing the flow of charge carriers during charging in the fluoride ion secondary battery according to the present invention. [Figure 2(B)] FIG. 2(B) is a schematic diagram showing the state of charge carriers during discharge in the fluoride ion secondary battery according to the present invention. [Figure 3(A)] FIG. 3(A) is a graph showing a cyclic voltammogram (solid line) of Zn2Mo(CN)8 according to Example 2 of the present invention. [Figure 3(B)] FIG. 3(B) is a graph showing a cyclic voltammogram of Zn2Mo(CN)8 according to Comparative Example 2. [Figure 3(C)] FIG. 3C is a graph showing a cyclic voltammogram of ZnFe(CN) 6 according to Comparative Example 3. [Figure 4] FIG. 1 is a schematic diagram of a sealed fluoride ion secondary battery according to a third embodiment of the present invention. [Figure 5] 10 is a graph showing the charge-discharge characteristics of a sealed fluoride ion secondary battery according to Example 3 of the present invention. [Figure 6] 10 is a graph showing a cyclic voltammogram of Fe2Mo(CN)8 according to Example 4 of the present invention. [Figure 7] 10 is a graph showing a cyclic voltammogram of Co2Mo(CN)8 according to Example 5 of the present invention. [Figure 8] 10 is a graph showing a cyclic voltammogram of Ni2Mo(CN)8 according to Example 6 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0040] Hereinafter, embodiments of a positive electrode active material according to the present invention and a fluoride ion secondary battery using the positive electrode active material will be described in detail with reference to the accompanying drawings.

[0041] [Embodiment Mode] <1. Cathode active material> FIG. 1(A) shows a compound of general formula M x This is a schematic diagram showing the molecular structure of metal complex (5) represented by Mo(CN)₈yHO. The metal complex contains M ions (3), molybdenum ions (2), and cyano groups (4). Fluoride ions (1), which act as charge carriers, are inserted into the gaps of the three-dimensional (jungle gym-like) structure of the metal complex (5). Generally, anions have a larger ionic radius than cations, making it difficult for them to be inserted into or removed from compounds. However, the fluoride ion (1) has an ionic radius similar to that of the potassium ion, making it possible for it to be inserted into compounds.

[0042] 1(B) is a diagram schematically illustrating the operation of a positive electrode containing a positive electrode active material for a secondary battery according to this embodiment. x Molybdenum (2) in the metal complex represented by Mo(CN)8·yH2O releases electron (6) to the electrode side, converting it from a tetravalent to a pentavalent (Mo 4+ →Mo 5+ ) and a fluoride ion is inserted into the metal complex.

[0043] 2. Carrier flow in fluoride-ion secondary batteries Figure 2(A) shows the flow of charge carriers, including the external circuit. Electrons (6) flow out of the positive electrode (11) into the external circuit, and simultaneously, fluoride ions (1) are inserted into the positive electrode active material. Electrons (6) released from the positive electrode (11) flow out of the positive electrode (11) into the external circuit and then flow into the negative electrode (12) via the external circuit. The movement of electrons (6) from the positive electrode (11) to the negative electrode (12) means that current (15) flows from the negative electrode (12) to the positive electrode (11). This corresponds to the state in which the secondary battery is being charged using an external power source (16).

[0044] In the fluoride ion secondary battery shown in Figure 2(A), a common metal zinc electrode is used for the negative electrode (12). Electrons (6) flowing into the negative electrode (12) from the external circuit combine with zinc ions (17) in the electrolyte or with zinc ions released from zinc fluoride ZnF2 (19) deposited on the surface of the negative electrode (12) to form zinc Zn. At the same time, fluoride ions (1) are released into the solution.

[0045] Figure 2(B) shows the flow of charge carriers, including the external circuit. Electrons (6) emitted from the negative electrode (12) flow out of the negative electrode (12) into the external circuit and then flow into the positive electrode via the external circuit. The movement of electrons from the negative electrode (12) to the positive electrode (11) means that current flows from the positive electrode (11) to the negative electrode (12). This corresponds to a state in which current flows from the secondary battery to the load (18).

[0046] In the fluoride ion secondary battery shown in Figure 2(B), electrons (6) are released from the metallic zinc electrode of the negative electrode (12) into the external circuit, and at the same time, zinc dissolves into the solution as zinc ions (17). The dissolved zinc ions (17) immediately combine with fluoride ions (1) in the solution to form zinc fluoride ZnF2 (19), which precipitates on the surface of the negative electrode. Because the solubility of zinc fluoride (19) is extremely low, this reaction occurs immediately.

[0047] 3. Sealed fluoride-ion secondary battery FIG. 4 shows a schematic diagram of a sealed fluoride ion secondary battery (20). The sealed fluoride ion secondary battery consists of a positive electrode (11), a negative electrode (12), and a separator (13). The positive electrode (11) is arranged at the top and the negative electrode (12) at the bottom, with a separator (13) between the positive electrode (11) and the negative electrode (12). The separator (13) is impregnated with an electrolyte (not shown). The positive electrode (11) and the negative electrode (12) are in contact with each other via the separator (13), and the positive electrode (11), the negative electrode (12), the electrolyte, and the separator (13) are all sealed in the same area by an exterior body (14).

[0048] <3-1. Positive electrode> The positive electrode (11) is a positive electrode mixture fixed to a conductive substrate. The positive electrode mixture is a compound represented by the general formula M x It is made from a positive electrode active material containing a metal complex represented by Mo(CN)8·yH2O, a conductive additive, and a binder.

[0049] <3-2. Conductive additives> Suitable examples of the conductive additive used in this embodiment include carbon materials such as carbon blacks, graphites, carbon nanotubes (CNTs), and vapor-grown carbon fibers (VGCFs). Examples of carbon blacks include acetylene black, oil furnace black, and ketjen black. The conductive additives may be used alone or in combination of two or more. There are no particular restrictions on the mixing ratio of the positive electrode active material and the conductive additive. However, the content of the conductive additive in the positive electrode is preferably 1% by mass to 500% by mass, more preferably 1% by mass to 300% by mass, even more preferably 1% by mass to 200% by mass, and particularly preferably 1% by mass to 150% by mass, relative to the total mass of the positive electrode active material contained in the positive electrode.

[0050] <3-3. Binder> The binder is not particularly limited, and examples of known binders include polymer compounds, and preferred examples include fluororesins, polyolefin resins, rubber polymers, polyamide resins, polyimide resins (such as polyamideimides), and cellulose ethers.

[0051] Specific examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), polyethylene, aromatic polyamide, cellulose, styrene-butadiene rubber, isoprene rubber, butadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymers, hydrogenated products thereof, styrene-ethylene-butadiene-styrene copolymers, styrene-isoprene-styrene block copolymers, hydrogenated products thereof, syndiotactic 1,2-polybutadiene, ethylene-vinyl acetate copolymers, propylene-α-olefin (having 2 to 12 carbon atoms) copolymers, starch, methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethylhydroxyethyl cellulose, nitrocellulose, polyacrylic acid, sodium polyacrylate, and polyacrylonitrile.

[0052] The binder may be used alone or in combination of two or more kinds.

[0053] The mixing ratio of the positive electrode active material and the binder is not particularly limited, but the content of the binder in the positive electrode is preferably 1 mass % to 50 mass %, and more preferably 1 mass % to 25 mass %, relative to the total mass of the positive electrode active material contained in the positive electrode.

[0054] <3-4. Positive electrode current collector> Examples of the positive electrode current collector include foils, meshes, expanded grids (expanded metals), punched metals, nonwoven fabrics, porous bodies, etc., made of conductive materials such as carbon paper, nickel, and stainless steel (SUS). There are no particular limitations on the mesh size, wire diameter, number of meshes, porosity, etc., and conventionally known materials can be used.

[0055] The shape of the positive electrode current collector is not particularly limited and may be selected according to the desired shape of the positive electrode, for example, a foil shape, a plate shape, or the like.

[0056] <3-5. Negative electrode> The negative electrode (12) may be any material that oxidizes and reduces at a potential lower than that of the positive electrode active material, such as a metal fluoride containing one or more metals selected from the group consisting of zinc (Zn), tin (Sn), and lead (Pb).

[0057] <3-6. Separator> The separator (13) serves to physically separate the positive and negative electrodes to prevent internal short circuits. The separator (13) is impregnated with an electrolyte and has fluoride ion permeability to ensure the battery reaction.

[0058] The separator (13) may be, for example, a resin porous membrane or a nonwoven fabric. The separator may be formed of only a porous membrane layer or a nonwoven fabric layer, or may be formed as a laminate of multiple layers with different compositions and forms. Suitable laminates include, but are not limited to, a laminate having multiple resin porous layers with different compositions, and a laminate having a porous membrane layer and a nonwoven fabric layer.

[0059] The material of the separator (13) can be selected taking into consideration the operating temperature of the battery, the composition of the electrolyte, and the like.

[0060] The resin contained in the fibers forming the porous membrane and nonwoven fabric may be one or more selected from the group consisting of derivatives such as cellulose and nitrocellulose, polyolefin resins such as polyethylene, polypropylene, and ethylene-propylene copolymer, polyphenylene sulfide resins such as polyphenylene sulfide and polyphenylene sulfide ketone, polyamide resins such as aromatic polyamide resins (e.g., aramid resin), and polyimide resins. The resin may be hydrophilized.

[0061] The shape and size of the separator (13) are not particularly limited and may be selected appropriately according to the desired shape of the battery.

[0062] <3-7. Electrolyte> The electrolyte solution is prepared by dissolving a fluoride salt in water. The water is preferably water from which impurities have been removed, such as ion-exchanged water or ultrapure water. The electrolyte solution must contain fluoride ions and must be neutral to weakly alkaline so that the positive electrode active material does not dissolve. The fluoride salt may contain at least one of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride. The concentration is preferably 1 mol / L to 13.5 mol / L, more preferably 5 mol / L to 13 mol / L, and even more preferably 10 mol / L to 13 mol / L. If the negative electrode active material dissolves in the electrolyte solution, the negative electrode active material compound may be saturated in advance.

[0063] The above fluoride salts are more readily available on the market and more stable in the atmosphere than hexafluorophosphates (LiPF6, NaPF6, KPF6) and tetrafluoroborates (LiBF4, NaBF4, KBF4) used in lithium-ion secondary batteries.

[0064] The fluoride ion secondary battery according to this embodiment uses water as a solvent. There is no need to use a non-aqueous electrolyte solution, such as an organic electrolyte solution. Therefore, even if heat is generated, the electrolyte solution will not ignite, making it highly safe. [Example]

[0065] <Example 1: Preparation of positive electrode active material> The positive electrode active material (Zn2Mo(CN)8) was synthesized as follows.

[0066] A molybdenum source solution, prepared by dissolving 10 g of homemade potassium octacyanomolybdate K4Mo(CN)8·2H2O in 200 mL of ultrapure water, was mixed with a zinc source solution, prepared by dissolving 60 g of zinc sulfate (ZnSO4·7H2O, Kishida Chemical) in 200 mL of ultrapure water. The resulting precipitate was washed with water, filtered, and dried to synthesize the positive electrode active material. This process is referred to as the Zn process.

[0067] In the above Zn process, Fe2Mo(CN)8 was synthesized by converting zinc sulfate ZnSO4·7H2O into iron sulfate FeSO4·7H2O.

[0068] In the above Zn process, Co2Mo(CN)8 was synthesized by changing zinc sulfate ZnSO4·7H2O to cobalt sulfate CoSO4·7H2O.

[0069] In the above Zn process, Ni2Mo(CN)8 was synthesized by converting zinc sulfate ZnSO4·7H2O into nickel sulfate NiSO4·6H2O.

[0070] Example 2: Evaluation of positive electrode containing positive electrode active material ZnMo(CN) A cyclic voltammogram was carried out for the Zn2Mo(CN)8 prepared in Example 1. The positive electrode was used as the working electrode, and a platinum counter electrode and a lead|lead fluoride reference electrode (hereinafter referred to as Pb|Pb 2+ A three-electrode electrochemical cell consisting of a Pb electrode and a Pb electrode was filled with a 10 mol / L potassium fluoride aqueous solution as the electrolyte, and a cyclic voltammogram was performed using a potentiostat manufactured by BioLogic (France). 2+ The charge-discharge characteristics were evaluated by examining the redox response in the voltage range from 1.0 V to 1.6 or 1.65 V. The results are shown by the solid line in FIG. 3(A).

[0071] Paired oxidation and reduction peaks were confirmed at approximately 1.46 V and 1.31 V. This indicates that the compound can be reversibly oxidized and reduced, i.e., can be charged and discharged.

[0072] <Comparative Example 1: Evaluation of Positive Electrode Not Containing Positive Electrode Active Material> A positive electrode was fabricated by applying a positive electrode mixture slurry that did not contain the positive electrode active material Zn2Mo(CN)8 to carbon paper. A cyclic voltammogram of the positive electrode was performed using an evaluation device with the same configuration as in Example 2. The results are shown by the dashed line in FIG. 3(A).

[0073] In this result, neither an oxidation peak nor a reduction peak was confirmed, and it was found that the oxidation peak and reduction peak confirmed in Example 2 were due to the positive electrode active material Zn2Mo(CN)8.

[0074] Comparative Example 2: Evaluation of a positive electrode containing a positive electrode active material ZnMo(CN) (when the electrolyte is a potassium sulfate aqueous solution) Cyclic voltammograms were performed using an evaluation device with the same configuration as in Example 2, except that a saturated potassium sulfate aqueous solution was used as the electrolyte and a mercury / mercury oxide electrode was used as the reference electrode. The results are shown in Figure 3(B).

[0075] In this result, a weak oxidation wave associated with the oxidation of water was observed, but a corresponding reduction wave was not observed. This indicates that the presence of fluoride ions is essential for the oxidation peak and reduction peak observed in Example 2.

[0076] Comparative Example 3: Evaluation of positive electrode containing positive electrode active material ZnFe(CN) One of the objectives of the present invention is to obtain a high output voltage for the secondary battery, and as mentioned above, the molybdenum complex ion framework [Mo(CN)8] coordinated with a cyano group was selected for this purpose. Therefore, it is necessary to verify whether the oxidation peak and reduction peak potentials obtained in Example 2 are attributable to the molybdenum framework. For this verification, as a comparative example, ZnFe(CN)6 was synthesized, which consisted of an iron complex ion framework [Fe(CN)6] coordinated with a cyano group instead of a molybdenum complex ion framework [Mo(CN)8] coordinated with a cyano group. Next, a cyclic voltammogram was performed using the same measurement conditions and evaluation equipment as in Example 2, except that the positive electrode active material Zn2Mo(CN)8 was changed to ZnFe(CN)6. The results are shown in Figure 3(C).

[0077] Paired oxidation and reduction peaks were observed at 1.09 V and 0.95 V. Compared to the case where the positive electrode active material Zn2Mo(CN)8 was used (Example 2), both the oxidation and reduction peak potentials were found to be 0.36 V lower. The oxidation and reduction peak potentials obtained in Example 2 indicate that they include the contribution of the molybdenum complex ion framework [Mo(CN)8] coordinated with cyano groups. In other words, by using a compound having Zn2Mo(CN)8 as the main structure, the redox potential of the positive electrode can be increased by 0.36 V compared to when a compound having ZnFe(CN)6 as the main structure is used. Furthermore, the results of this experiment show that when this molybdenum complex ion framework is used, a higher output voltage of the secondary battery can actually be obtained compared to when other cyano-coordinated complex ion frameworks are used.

[0078] <Example 3: Fabrication of sealed fluoride ion secondary battery> The method for producing a sealed fluoride ion secondary battery is described below.

[0079] A positive electrode mixture slurry was prepared by mixing 40 parts by weight of ZnMo(CN) as the positive electrode active material, 50 parts by weight of Denka Black (Denka Co., Ltd.) as the conductive additive, and 10 parts by weight of polyvinylidene fluoride (Kureha Corporation, #1100) as the binder in n-methylpyrrolidone solvent (3 times the weight of the solid content). The positive electrode mixture slurry was applied to a carbon paper (Toray Industries, Inc., TGP-H090) as the positive electrode current collector and dried to prepare a positive electrode.

[0080] The negative electrode was prepared by fixing zinc metal foil to carbon paper as a negative electrode current collector.

[0081] The positive electrode and negative electrode prepared by the positive electrode preparation procedure were laminated together with a hydrophilized polyolefin nonwoven fabric separator, and a 10 mol / L potassium fluoride aqueous solution (saturated with zinc fluoride) as an electrolyte was sealed in the laminate film to prepare a sealed fluoride ion secondary battery. A schematic diagram of a sealed fluoride ion secondary battery is shown in Figure 4.

[0082] A charge / discharge test was carried out on the obtained sealed fluoride ion secondary battery using a potentiostat (described above). The results are shown in Figure 5.

[0083] The intermediate charge voltage obtained from the charge curve was 1.6 V, the intermediate discharge voltage obtained from the discharge curve was 1.15 V, and the discharge capacity per gram of positive electrode active material was 40 mAh / g.

[0084] Example 4: Evaluation of positive electrode containing positive electrode active material FeMo(CN) A cyclic voltammogram of Fe2Mo(CN)8 was carried out using the same evaluation equipment and conditions as in Example 2, except that the positive electrode active material Zn2Mo(CN)8 was changed to Fe2Mo(CN)8. The results are shown in Figure 6.

[0085] A pair of oxidation peaks with a maximum at 1.37 V and reduction peaks with a plateau between 1.2 V and 1.3 V was confirmed. It was found that Fe2Mo(CN)8 can be reversibly oxidized and reduced, i.e., can be charged and discharged.

[0086] Example 5: Evaluation of positive electrode containing positive electrode active material CoMo(CN) A cyclic voltammogram of Co2Mo(CN)8 was carried out using the same evaluation equipment and conditions as in Example 2, except that the positive electrode active material Zn2Mo(CN)8 was changed to Co2Mo(CN)8. The results are shown in Figure 7.

[0087] An oxidation peak with two maxima at 1.35 V and 1.48 V and a reduction peak with a maximum at 1.31 V were confirmed. This indicates that Co2Mo(CN)8 can be reversibly oxidized and reduced, i.e., can be charged and discharged.

[0088] Example 6: Evaluation of positive electrode containing positive electrode active material NiMo(CN) A cyclic voltammogram of Ni2Mo(CN)8 was carried out using the same evaluation equipment and conditions as in Example 2, except that the positive electrode active material Zn2Mo(CN)8 was changed to Ni2Mo(CN)8. The results are shown in Figure 8.

[0089] A pair of oxidation peaks with a maximum at 1.47 V and reduction peaks with a maximum at 1.39 V was confirmed. It was found that Ni2Mo(CN)8 can be reversibly oxidized and reduced, that is, charged and discharged. [Industrial Applicability]

[0090] The present invention provides a positive electrode active material and a fluoride ion secondary battery using the active material, which can provide a satisfactorily high output voltage, can be charged immediately after production, can be constructed using a negative electrode material that is available on the market, and has a high level of safety, capable of preventing the electrolyte from igniting. [Explanation of symbols]

[0091] 1. Fluoride ion 2. Molybdenum ions 3 M ions 4 Cyano group 5M x Mo(CN) 8· Metal complex represented by yH2O 11 Positive electrode 12 Negative electrode 13 Separator 20 Sealed fluoride-ion secondary battery

Claims

1. A positive electrode active material for a secondary battery containing an aqueous electrolyte, The general formula is M ｘ Mo (CN) ８ ・yH ２ 0, wherein in the general formula, M is one element selected from the group consisting of Fe, Co, Ni, and Zn, and x and y satisfy the relationships 1.95<x<2.05 and 0≦y≦9, respectively, and the positive electrode active material for a secondary battery is in an oxidizable state, i.e., a chargeable state, in an electrochemical reaction using a fluoride ion as an intercalating species.

2. A fluoride ion secondary battery using the positive electrode active material for secondary batteries according to claim 1, wherein the fluoride ion secondary battery comprises: a positive electrode, a negative electrode, the aqueous electrolyte solution, and a separator; the positive electrode contains the positive electrode active material for a secondary battery, the aqueous electrolyte is an aqueous solution in which a fluoride salt is dissolved, The separator is provided between the positive electrode and the negative electrode. A fluoride ion secondary battery characterized by:

3. 3. The fluoride ion secondary battery according to claim 2, wherein the battery is a sealed battery prepared by impregnating the separator with the aqueous electrolyte solution.

4. 4. The fluoride ion secondary battery according to claim 2, wherein the fluoride salt is at least one selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, and cesium fluoride.

Images